The invention relates in general to the field of electronic color imaging. The invention relates more specifically to electronic imaging devices that utilize a single multi-color image sensor, incorporating a color filter array, to generate both luminance and color information.
It is desirable in the field of electronic imaging to compress electronic image data to reduce the amount of storage space required to store a captured image. In electronic still cameras, for example, image cards have been proposed to store the electronic image data generated by the image sensor of the camera. Reductions in the size and expense of the image cards can be achieved, while at the same time increasing their image storage capability, if electronic image compression techniques are employed to reduce the amount of actual image data that must be stored to subsequently reproduce each image in detail.
Standards for performing image compression have recently been developed. See, for example, JPEG Draft Technical Specification JPEG-8-R5 (January 1990). In addition, devices for performing image compression in accordance with the developed standards, for example the CL550B JPEG Image Compression Processor, available from C-Cube Microsystems, have recently become available. The available devices accept a raster stream of image data, including up to three channels of color information, and then perform all of the necessary formatting tasks to form 8.times.8 image blocks (for example with the use of eight line buffers) to which further image processing functions are applied.
Using three separate color channels, however, is not desirable, as separate red, green and blue image data (RGB) would have to be generated for each pixel of the image to be captured. If a single sensor with a color filter array is used, this requirement makes the input data three times as large as the sensor resolution. It is possible, however, to utilize a separate luminance channel (G) and to interleave separate chrominance channels (R-G, B-G) into a single chrominance channel in order to reduce the amount of data that must be generated. In such a case, a single image sensor incorporating a color filter array (CFA) could be used to generate the required luminance and interleaved chrominance channel.
The use of single image sensors incorporating CFAs to generate both luminance and color information is well known. Red, green and blue color information is isolated by passing scene light through a color filter having a specific pattern of red, green and blue filter elements before the scene light strikes a photosite sensor element of the image sensor. Information corresponding to all three colors is therefore not measured for each discrete pixel of the image. Instead, information from neighboring pixels must be used, generally through an interpolation process, in order to generate luminance and color differences for each image pixel.
A problem exists with conventional CFA patterns, however, when attempting to create the interleaved chrominance channel described above, namely, actual red and blue values required to generate the interleaved chrominance channel are not always available to match the B-G, R-G, B-G chrominance channel interleave pattern. Interpolated red and/or blue values must therefore be used in place of actual values, which introduces a significant amount of error into the system. It should be noted that interpolation of a red or blue value introduces a higher degree of error than the interpolation of a green value, as the red and blue colors, which are used to generate the chrominance signal, are usually sampled at a much lower frequency than green, which is used to produce the luminance channel.
In view of the above, it is an object of the invention to provide a CFA that can be incorporated in an electronic image sensor that does not require the use of interpolated red and blue values to generate an interleaved chrominance channel.